Most people who request weight-gaining information are adolescents who are going to naturally gain weight with age anyway. Increasing lean body mass is the key to gaining healthy weight. Concentrating on high-calorie, fatty foods will increase body fat, but that is not necessarily desirable...even for the very thin.

Gaining lean body weight involves increasing muscle mass, whether or not you are interested in obtaining a muscular frame. Gaining lean weight through weight training can translate into becoming more muscular, but increasing muscle size is only achieved through directed, specific training to increase mass and definition.

For adults who desire weight gains, both short and long-term benefits exist for increasing overall strength through weight training for lean mass in addition to eating a proportionate amount of healthy foods based on present height & weight and the desired amount of weight gain. Though benefits have been shown from adding protein powders and various supplements into your diet, they are not necessary for weight gain (See Question #5).

The Bottom Line:
As mentioned in "How do I lose fat & extra weight", (see Question #4) calories in and calories out determine your weight and whether you will gain or lose. Eat more lean meats (fish & chicken), take in more lower fat dairy or dairy substitutes, increase your strength training routine, and substitute more strength exercises for cardio workouts if you are cardio training over 3 times per week for an hour or more.